A variety of processes for alkylating aromatics using conventional aluminosilicate molecular sieve catalysts are commercially available. Various aromatic compounds are either naturally present in or are traditionally produced from petroleum feedstock by catalytic reforming processes.
Aromatics alkylation is an important procedure for producing many useful chemical products. For example, para-xylene, which can be produced by alkylating toluene with methanol, constitutes an important starting material for manufacturing synthetic polyester fibers, films, and resins. These polyester materials have many practical, well known uses, such as in fabrics, carpets, and apparel. Other alkylated aromatics have similar roles.
Methanol, the preferred alcohol for para-xylene production from a toluene feedstock, is typically synthesized from the catalytic reaction of hydrogen, carbon monoxide and/or carbon dioxide in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one synthesis process methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor.
Molecular sieves are porous solids having pores of different sizes including crystalline molecular sieves such as zeolites, as well as carbons and oxides. The most commercially useful molecular sieves for the petroleum and petrochemical industries are crystalline molecular sieves. Crystalline molecular sieves in general have a one-, two-, or three-dimensional crystalline pore structure having uniformly sized pores of molecular scale within each dimension. These pores selectively adsorb molecules that can enter the pores and exclude those molecules that are too large.
Examples of some potentially useful molecular sieves for aromatics alkylation include aluminosilicate molecular sieves as described in co-pending U.S. patent application Ser. No. 09/866,907 (ITQ-13) and in U.S. Pat. No. 3,702,886 (ZSM-5), U.S. Pat. No. 4,076,842 (ZSM-23), U.S. Pat. No. 4,397,827 (ZSM-48), and U.S. Pat. No. 4,954,325 (MCM-22), all of which are herein fully incorporated by reference. Aluminosilicate molecular sieves, also known as zeolites, contain a three-dimensional microporous crystalline framework structure of [SiO4] and [AlO4] corner sharing tetrahedral units. Zeolites are generally synthesized by the hydrothermal crystallization of a reaction mixture of silicon and aluminum sources. Other metallosilicate molecular sieves with various metals (such as, for example, gallium, iron, and/or boron) substituted for aluminum in some portion of the crystalline framework are also known in the art.
Aluminum and phosphorus containing molecular sieve crystals (for example, ALPO and SAPO) can be produced by the hydrothermal crystallization of a reaction mixture of silicon, aluminum, and phosphorus sources along with at least one templating agent as described, for example, in U.S. Pat. No. 4,440,871, which is herein fully incorporated by reference.
Molecular sieves are often formed into molecular sieve catalyst compositions to improve their durability and to facilitate handling in commercial conversion processes. These molecular sieve catalyst compositions are formed by combining a molecular sieve with a matrix material and/or a binder. Although the use of binders and matrix materials are known for use with molecular sieves to form molecular sieve catalyst compositions useful in alkylating aromatics, these binders and matrix materials typically only serve to provide desired physical characteristics to the catalyst composition and have little to no effect on conversion and selectivity of the molecular sieve.
Many of the toluene alkylation processes use catalytic materials which are prone to rapid catalyst deactivation, usually due to coke formation, under typical reaction conditions and, therefore, require constant regeneration. This regeneration requirement usually necessitates the use of higher cost technology such as fluid bed reactors wherein the catalyst is continuously regenerated.
Although a variety of treatments have been disclosed for improving conversion, improving product selectivity, and/or reducing coke formation, there is still a significant problem with rapid catalyst de-activation due to coke formation at the reaction conditions required for alkylation of aromatics. It would therefore be desirable to have an improved molecular sieve catalyst composition having longer lifetimes and, preferably, also having better conversion rates, product selectivity, and commercially desirable operability and cost advantages.